Utility manholes for storm water, sewer water and like underground utility systems are situated at periodic intervals along the length of the fluid handling system, both periodically along straight lengths and at bends in the system, to provide a network of access points into the system from above ground. Such manholes are typically cast of concrete and may be fabricated on-site or, more preferably and more typically in the current state-of-the-art, precast in a factory setting and transported to an installation site.
Manholes are conventionally constructed of a round outer housing, i.e., circular in horizontal cross-section, within which a formed concrete basin defines a water flow trough, commonly referred to as an invert, between space-apart lateral openings in the housing. The incoming and outgoing openings in the manhole housing may be aligned with one another for straight-through water flow, or may be disposed at angles to one another, usually no less than ninety degrees)(90°, for a transition in flow direction. The trough connecting between the openings must have a curvature providing a smooth transition through the angular turn.
The size of manholes is an important factor in the design and construction of a utility system. The larger the manhole, the more difficult is the installation and the greater is the cost. Large manholes may not be possible to pre-fabricate off-site, requiring more expensive on-site in-situ fabrication. Conventionally, the size of a manhole is dictated as a function of the size of the pipe connecting into the incoming and outgoing sides of the manhole. The outer diameter of the manhole housing must be large enough and the wall thickness of the manhole housing must be thick enough to allow for the formation of recessed countersunk openings, each with a flat stop face, to accommodate the diameter of the incoming and outgoing pipes and thereby to facilitate secure sealed connection of the pipes to the manhole housing. Thus, smaller diameter round manholes, because of the curvature thereof, cannot accommodate incoming and outgoing large diameter pipes, even though the interior space within the manhole may be able to accommodate the necessary trough curvature.